With the international phase-out of chlorofluorocarbons (CFCs) and hydrochlorofluorocarbons (HCFCs) as refrigerants under the requirements of the Montreal Protocol, a variety of refrigerant compositions that do not cause depletion of the ozone layer have been proposed.
However, following the European Community's ratification of the Kyoto Protocol, attention has turned from these CFC and HCFC based refrigerants (whose use has now all but ceased within the EU and other developed countries) to HFCs and other refrigerants that emerged as alternatives to CFCs in the 1990s. Whilst these other refrigerants have no or low ozone depleting potential (ODP), they suffer from the drawback that many of them have a high GWP.
To help the EU to meet its obligations under the terms of the Kyoto treaty, the European Parliament has recently introduced a directive and regulation relating to the use and emission of such fluids.
EC Regulation No 842/2006 introduces a number of controls and restrictions for the use of HFCs in a number of applications. Directive 2006/40/EC goes one step further and prohibits the use, in vehicle air-conditioning systems, of certain fluorinated greenhouse gases with a GWP higher than 150 (evaluated over the standard 100-year time horizon), unless the rate of leakage is no more than 40 to 60 grams of fluorinated greenhouse gases per year.
Whatever the specifics of these items of legislation, it appears likely that new restrictions will be identified for HFCs and other high GWP fluids, and that the use of these fluids will gradually be phased-out for new refrigeration, air conditioning and heat pump applications, and possibly for the refilling of existing systems.
To prepare for such eventualities it would be prudent to investigate practical alternatives to high GWP refrigerants, particularly those alternatives that maintain the advantages associated with some of the existing fluids, namely a low normal boiling point (NBP2) in the order of approximately −80° C. to −50° C. In general terms, systems built round refrigerants with an NBP in this range tend to be more compact, and have the potential to be more efficient than systems built round refrigerants with higher NBPs. There are also technical advantages associated with operating above atmospheric pressure, since this greatly reduces the possibility of contaminants such as air and moisture from being drawn into the system, which would result in reduction of refrigerating capacity, degradation of system efficiency and compromise long-term reliability. 2 The NBP is the boiling point of a fluid at standard atmospheric pressure (101.325 kPa).
Whilst it would clearly be advantageous to use low-GWP refrigerants with NBPs in this range, it is unfortunately the case that the only fluid that is acceptable for use as a refrigerant, which exhibits isothermal or near-isothermal phase-change, has low-GWP (i.e. has a GWP of less than 150) and has an NBP in this range is, at least insofar as we are aware, carbon dioxide (R744). However, it also possesses certain characteristics that make it less desirable as a refrigerant, particularly its high triple point and low critical point.
Mixtures or blends of refrigerants could potentially provide an alternative, but most mixtures which achieve these criteria are zeotropes3 with an unacceptably high temperature glide, i.e., the change of phase under steady-flow conditions (such as in a direct expansion evaporator or condenser of a refrigerating system) is non-isothermal. 3 A Zeotrope is defined in International Standard, ISO 817: 2004 “Refrigerants—Designation and Safety Classification” as being: a blend composed of two or more refrigerants whose equilibrium vapour and liquid phase compositions are not the same at any point. An azeotrope is defined in the same international standard as: a blend composed of two or more refrigerants whose equilibrium vapour and liquid phase compositions are the same at a given pressure, but may be different at other conditions.
The use of an azeotropic refrigerant would have specific benefits over zeotropic mixtures, particularly since zeotropes exhibit properties that may ultimately contribute negatively to cycle efficiency of a system using a zeotropic mixture.
For example, with a zeotropic refrigerant fractionation, or partial separation, of refrigerant components may occur, and this may manifest itself as composition variations in circulating refrigerant. This fractionation can also result in disproportionate amounts of refrigerant components being released from the system in the event of a leak, thereby altering the original composition of the circulating refrigerant mixture.
Another negative contributor is the fact that heat exchanger performance in such a system would be reduced, both by temperature glides in the evaporator and condenser and also due to the fact that additional thermodynamic losses manifest as reductions in refrigerant heat transfer coefficient relative to that expected from the individual refrigerant components.
Yet another disadvantage is that system design and selection of system mechanical parts would be significantly more complex, and as a consequence optimisation of such a system would be more difficult and less precise.
It is likely that there would also be significant practical problems associated with a system using a zeotropic refrigerant. For example, interpretation of system performance by service and maintenance technicians would be more complex (such as interpretation of operating pressures and temperatures), and steps may have to be taken to avoid uneven frosting of some evaporators.
In the light of the foregoing, azeotropic or near azeotropic blends (that is to say, zeotropes with less glide than would adversely affect the proper functioning of a system employing that refrigerant, for example a glide of less than 2 K) with a low GWP, low environmental impact, and an NBP in the aforementioned range could prove to be highly desirable, particularly if legislation should emerge to ban the use of high GWP fluids such as HFCs.
However, identifying such blends is not a simple task as there are many fluids that could potentially give rise to an azeotropic blend; and there are many thousands of binary, tertiary, and higher order blends of these azeotropes that each could potentially be of interest.
Another significant problem is that it is not simply a matter of selecting individual azeotropes or near azeotropes with a low GWP and favourable thermodynamic characteristics as potential candidates for a blend, as the properties of the blends are often quite different from the properties of the individual components of the blends.
Another problem is that low GWP and favourable thermodynamic characteristics are not the only factors to consider when developing blends. Rather, a multitude of other factors (including: solubility with oils, critical temperature, cost, toxicity, triple point, temperature glide, flammability, ODP) should also be considered when contemplating the use of such fluids in refrigeration and other heat transfer systems. Particular attention should also be paid to the efficiency—or potential coefficient of performance (COP)—of the blend, as these factors are important if a system designed around that blend is to operate efficiently and hence have a reduced impact on the environment.
It is apparent from the foregoing that it would be highly advantageous if refrigerant blends could be devised that had a low GWP, a low environmental impact, an NBP in the aforementioned range, and which exhibit at least a good proportion of the following properties: favourable thermodynamic and transport properties, good solubility with oils, high critical temperature; low cost; low toxicity; low triple point; low temperature glide; low-flammability; low GWP; zero ODP; and high COP. It would also be highly advantageous if blends could be found that not only exhibit a good proportion of these properties, but are also environmentally benign and have good chemical and material compatibility.